Currently, the constant current source circuit has been widely used as circuits demanding a high current stability.
The constant current function is realized by a transistor T2 in the prior art (see FIG. 1). Since VBE of the transistor T2 is 0.7V, a sample voltage passing through a resistor R1 is about 0.6V. A final stable current is I=VBE/R1. The main drawbacks of this circuit are a poor current precision and a high thermal drift, because the transistor has a big VBE range (0.58-0.75V) and a high thermal drift.
The constant current source circuit is realized by a three-terminal shunt regulator U1 in the prior art (see FIG. 2). As shown in FIG. 2, the constant current source circuit comprises the three-terminal shunt regulator U1 and a transistor T1. The three-terminal shunt regulator U1 has an output terminal 2 connected to a voltage input terminal VIN through a resistor R3, an adjusting terminal 1 connected to a node between a sample resistor R1 and an LED as a load, and an input terminal connected to a reference potential. The resistor R3 is connected between a base of the transistor T1 and the voltage input terminal VIN, and the transistor T1 has an emitter connected to the voltage input terminal VIN and a collector connected to the load. The three-terminal regulator U1 may be TL431. As the TL431 is used for the constant current, an output current is controlled by controlling U1 (TL431). As a conduction voltage of TL431 is 2.495V, a voltage passing through the sample resistor is 2.495V, and the power consumption at this time is P=2.495*I. The main drawback of this solution is big power consumption as the sample voltage is as high as 2.495V, and the advantage is a high precision and a low thermal drift.
In the two solutions above, voltages at both ends of the sample resistor are close to a forward voltage drop VF (generally, 0.7-3.1V) of the LED as the load when enabled, resulting big power consumption.